The present invention generally relates to power control for wireless transmitters, and more particularly to variable gain amplifier circuits for wireless transmitters.
Wireless communication systems use power control to reduce interference and increase system capacity while maintaining minimum signal quality standards. The capacity of wireless communication systems, i.e., WCDMA (Wideband Code Division Multiple Access) systems, relies heavily on the accurate implementation of uplink power control. Many wireless communication standards, such as 3GPP TS 25.101, include specific requirements for transmit power control accuracy within the wireless communication device. These device requirements include both absolute and relative accuracy transmit power requirements. The absolute requirements define a lower and an upper transmit power limit relative to a nominal transmit power. The relative requirements define minimum and maximum transmit power differences between two transmitted time slots, not necessarily adjacent time slots, as well as an aggregated transmit power difference over several time slots.
Closed-loop power control represents one method for controlling the transmit power within the wireless communication device to comply with the relative and absolute transmit power requirements. As used herein, closed-loop power control refers to feedback power control implemented within the wireless communication device. A closed-loop power control system determines the error between a measured transmit power and a desired transmit power. Based on this error, the closed-loop power control system adjusts the transmit power by adjusting a gain of a variable gain amplifier within a wireless transmitter of the wireless communication device.
Because power detectors have a limited dynamic range, wireless communication devices may be unable to accurately measure low transmit powers, causing closed-loop power control to become unreliable at low transmit powers. To avoid this, the wireless communication device may alternatively use open-loop power control. As used herein, open-loop power control refers to power control implemented within the wireless communication device that adjusts the transmit power, responsive to a power control command, based on known device operation parameters and/or environmental conditions. Open-loop power control enables compliance with the relative power requirements. However, because open-loop power control does not include any means for verifying the accuracy of the transmit power, open-loop power control may generate transmit powers that drift away from the desired transmit power, and therefore, violate the absolute power requirements.
Another solution may use a combination of closed-loop and open-loop power control. When a measured transmit power meets or exceeds a predetermined threshold, the wireless communication device implements the closed-loop power control. Otherwise, the wireless communication device implements open-loop power control.
It will be appreciated that this combination solution generally addresses the above-discussed issues associated with pure closed-loop and pure open-loop power control systems. However, because the open-loop transmit power is relatively undefined before the switch to closed-loop, the power step that occurs when switching from open-loop to closed-loop power control will also be relatively undefined, which may cause a discontinuity to occur during the transition. Further, because a gain control signal during closed-loop power control may significantly differ from the gain control signal generated after switching from closed-loop to open-loop power control, a discontinuity may also occur when switching from closed-loop to open-loop power control. These discontinuities may cause the power step between adjacent time slots to exceed the relative transmit power requirements during transitions between open-loop and closed-loop power control. Therefore, transitions between open-loop and closed-loop power control must be carefully controlled to ensure compliance with the relative transmit power requirements.